1. Field of the Invention
The present invention relates to a driver circuit for driving a semiconductor laser diode.
2. Related Prior Art
Along with the diffusing of the internet, the optical communication becomes very popular in various fields to transmit huge information and the requirements for the optical transmitter becomes complex and diverse. For example, the trunk line in the telecommunication or the large area network (LAN) request the high speed modulation to transmit the huge information, while the access network is necessary for the bust mode communication to realize the one-to-N (1:N) protocol. Generally, the laser driver for driving the semiconductor laser diode is optimized in the individual communication protocol. In the application for the high speed communication, in order to prevent the degradation of the signal quality due to the impedance mismatching between the semiconductor laser and the laser driver, the laser driver adjusts the output impedance thereof with the transmission impedance by providing a termination resistor to reduce the reflection of the high frequency signal at the output as shown in the Japanese patent published as 2004-193489A and 2004-047832A. The termination resistor for the impedance matching is necessary to be connected immediately to the collector of the output transistor of the laser driver. Particularly, it is quite preferable for the termination resistor to be built within an integrated circuit of the laser driver for the application that requires the high speed modulation. For the application that requires the burst mode transmission, the laser diode is necessary to be directly coupled with the laser driver. When the modulation speed is slow such that the reflection due to the impedance mismatching between the laser diode and the laser driver does not degrade the signal quality, it is usual to omit the termination resistor in the output terminal to match the impedance with the transmission line and the laser diode in order to reduce the power dissipation by eliminating the void current.
Thus, the optimized configuration of the output characteristic, in particular, the output impedance of the laser driver depends on each application. The application in the high speed modulation is necessary to build in the termination resistor for the impedance matching, while, the application in the low speed and the lower power dissipation, or the application for the burst mode transmission, the termination resistor is preferable to be taken off. Conventionally, the laser driver for the high speed application and that for the lower speed application are individually prepared.
The present invention is to provide a laser driver in which the configuration for the high speed application and that for the lower speed application may be selectable.